This invention relates to a method for reducing artifacts due to projection measurement inconsistencies at the beginning and end of a scan. More specifically, the invention relates to such method particularly suitable for use with imaging modalities such as, for example, transmission-computed tomography (CT), emission-computed tomography, nuclear magnetic resonance (NMR), and ultrasound.
Each of the aforeidentified imaging modalities may utilize sets of projection data obtained at a plurality of projection angles through a transaxial slice of an object undergoing examination. The projection data are used to reconstruct images of the slice. The preferred embodiments of the invention utilizing divergent ray fan beam and parallel ray beam projections will be disclosed herein with reference to medical diagnostic applications of transmission-computed tomography and emission-computed tomography, respectively. Such images also find uses in many non-destructive testing applications which are contemplated within the scope of the present invention. As used herein, the term "transmission-computed tomography" refers to measurement of radiation transmitted through an object and will also be referred to hereinafter as CT. Emission-computed tomography refers to measurement of radiation emitted from within the object by, for example, radio-pharmaceutical isotopes.
In the simplest of scan geometries, the projections are obtained by measuring the transmission or emission of radiation along parallel ray paths. Early CT apparatus utilized a radiation source collimated to produce a "pencil" beam of radiation with a single detector to detect radiation not absorbed or scattered by the object. A projection was obtained by the joint translation of the source and detector so as to linearly scan the object. The beam, intensity modulated by the internal features of the object intercepting the radiation, was detected and converted to an electrical signal of corresponding intensity. After a first pass across the object, the angle of the beam (hence, that of the projection) was rotated by a small amount relative to the object and the translation repeated. The process was continued so as to obtain projections or views covering at least 180.degree. of an arc. A large number of current readings were made at a large number of points during the linear translate motion.
Emission-computed tomography can also be performed using a translate-rotate technique. Briefly, a single collimated detector would translate and rotate and thereby measure the emission of radiation from the body along sets of parallel rays. It is more common for a scintillation camera equipped with a parallel hole collimator to be used. The collimator defines the parallel rays. The camera is made to rotate about a stationary object. In this way, the translation motion is eliminated and only rotation through the set of angles is necessary to obtain a plurality of projections, as will be more fully disclosed hereinafter.
In more sophisticated systems, such as in most current CT apparatus, the parallel ray beam is replaced by a single fan beam of radiation composed of divergent rays which are detected simultaneously by a plurality of detectors. In one preferred CT scan geometry, the source and detectors are made to orbit jointly about an isocenter to obtain projections for a full 360 degrees. Fan-beam scanning may also be employed in emission-computed tomography and in ultrasound imaging.
In NMR, parallel-ray projections are obtained by varying the directions of magentic-field gradients within a transverse slice of an object to be studied. With each gradient direction corresponding to a projection, the gradient directions are varied to obtain projections from at least 180.degree. of an arc. A detailed exposition of NMR principles may be found in "Nuclear Magnetic Resonance Imaging in Medicine," Igahu-Shoin, L. Kaufaman, et al, Editors, and in "NMR Imaging in Biomedicine," Academic Press, P. Mansfield and P. G. Morris.
The projection data obtained by any of the aforementioned methods are processed with the aid of a digital-computer means in accordance with techniques well known to the art to produce the desired transverse images. A preferred reconstruction technique employs convolution and back projection of the data. A detailed description of this and other suitable reconstruction techniques is provided by Brooks and Di Chiro in "Principles of Computer-Assisted Tomography (CAT) and Radiographic and Radioisotopic Imaging" Phys. Med. Biol., Volume 21, No. 5, pages 689-732, 1976.
A problem frequently encountered in reconstructing images from projections is that, regardless of the modality used or the particular beam geometry employed to obtain the projection data, a finite period of time elapses between the acquisition of the first projection and the last projection. If, during this interval, the object moves continuously, most projections will be consistent with their neighbors, but the first and last views will be inconsistent. As a result, streak artifacts pointing to the position of the radiation source (in the case of CT) when the first and last projections were measured will appear in the reconstructed image. Even if the object moves abruptly, some inconsistency between the first and last projections may be present, and streaks in the direction of the first/last projection (in addition to other motion artifacts) may be visible. Such artifacts in the reconstructed images may be also due to subtle error in the rotational motion of the scanner or mechanical stability of the device.
A related problem can arise in practical scanning machines used to collect projection data. In CT machines, or example, if the detector and X-ray tube are not rigidly mounted on the rotating member, there may be a gravitationally induced relative motion between them and mechanical hysteresis. This undesired component of hysteresis will cause projection inconsistencies which in turn can produce streak artifacts in the image much like those due to object motion. In NMR machines, a similar effect can be caused by non-linearities in the gradient magnetic fields and imbalance between the orthogonal components.
Therefore, as used herein the meaning of the term "motion" should not be limited to the case of motion of the scanned object alone, but should be broadened to include any relative motion effects which result in ray inconsistencies between the first and last views of the scan.
It is, therefore, an object of the invention to provide a method for reducing the effects of motion-related artifacts in images reconstructed from projections having inconsistencies therein.
It is another object of the invention to provide a method for reducing the effects of motion-related artifacts in the reconstructed image by recognizing that projections measured during a substantially complete rotation contain some redundant information and then using such information to diminish the inconsistency by a suitable means of combining the redundant data.
It is a further object of the invention to provide a method for reducing the effects of motion-related artifacts in images reconstructed from projections which is applicable independently of the modality or beam geometry used to obtain the projections.